SAM7X256_14 Datasheet, PDF (255/662 Pages) ATMEL Corporation – ARM-based Flash MCU

SPI_SR is set. As long as this flag is set, data is loaded in SPI_RDR. The user has to read the status register to clear the
OVRES bit.
When a transfer starts, the data shifted out is the data present in the Shift Register. If no data has been written in the
Transmit Data Register (SPI_TDR), the last data received is transferred. If no data has been received since the last
reset, all bits are transmitted low, as the Shift Register resets at 0.
When a first data is written in SPI_TDR, it is transferred immediately in the Shift Register and the TDRE bit rises. If new
data is written, it remains in SPI_TDR until a transfer occurs, i.e. NSS falls and there is a valid clock on the SPCK pin.
When the transfer occurs, the last data written in SPI_TDR is transferred in the Shift Register and the TDRE bit rises.
This enables frequent updates of critical variables with single transfers.
Then, new data is loaded in the Shift Register from the Transmit Data Register. In case no character is ready to be
transmitted, i.e. no character has been written in SPI_TDR since the last load from SPI_TDR to the Shift Register, the
Shift Register is not modified and the last received character is retransmitted.
Figure 28-9 shows a block diagram of the SPI when operating in Slave Mode.
Figure 28-9. Slave Mode Functional Block Diagram
SPCK
NSS
MOSI
SPIEN
SPIENS
SPIDIS
SPI_CSR0
BITS
NCPHA
CPOL
LSB
SPI
Clock
SPI_RDR
RD
RDRF
OVRES
Shift Register
MSB
MISO
SPI_TDR
TD
TDRE
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
255